Evidence for a human humoral immune response to HCV infection providing at least partial protection is accumulating. The observation that antibodies induced to a hypervariable region on HCV E2 envelope protein designated HVR1 have in vitro and in vivo virus neutralization activity indicates that protective antibodies can be elicited. However, antibodies to HVR1 are highly restrictive and are a contributing factor in the development of escape mutants. The primary goal of this project is to develop potent neutralizing human monoclonal antibodies (HMAbs) that could ultimately be used in immunoprophylaxis and potentially for the treatment of HCV infection. We hypothesize that the majority of antibodies with the broadest and most potent activities will recognize conformational epitopes. Preliminary studies indicate that selected individuals with HCV genotype 2a infection are more likely to mount a protective antibody response. It is the intent of this project to focus on the production of HCV-HMAbs to E2 from these selected individuals. More importantly, the recent development of a small animal model for acute HCV infection will permit an eff aboutcient means to test for virus neutralization. A unique irnmunocompromised mouse has been developed capable of sustaining human liver tissues infected with HCV under the renal capsule for up to 26 days. During this period, viral replication occurs as measured by increasing serum viral load. Preliminary studies with three HCV-HMAbs (of which 2 are to conformational epitopes on HCV E2) injected by intraperitoneal route 17-20 days post-transplant of infected tissues significantly reduced HCV viral load in a dose-dependent manner. The aims of the proposal are to generate HMAbs to HCV E2 protein from peripheral B-cells of individuals infected with genotype 2a; test selected antibodies in a novel human-mouse chimera small animal model for in vivo protection; and determine the breadth of reactivity of these antibodies to divergent HCV isolates.